1. Field
The method of the present invention relates to processes for in-situ retorting of tar sand formations and the simultaneous recovery of liquid and gaseous by-products. Tar sand refers to a consolidated or unconsolidated sedimentary rock in which the available pore space is filled to a varying extent with a viscous, semi-solid tar or bitumen.
2. State of the Art
The huge deposits of tar sands in the Western United States and Canada have stimulated activity by industry to devise practical and economical methods of recovery. Of the total reserves of tar sands in the United States (&gt;28 billion barrels) and Canada, (&gt;1300 billion barrels) less than fifteen percent (15%) are amenable to surface recovery. Therefore, the method of the present invention addresses the need for effective and cost-efficient in-situ processes for recovery of the major portions of these deposits.
Where in-situ recovery of gaseous and liquid products from tar sand formations have been heretofore described in the literature and U.S. patents, such processes have all involved penetration of the target formation by drilled vertical wellbores which are arranged in a suitable fashion and have generally included initiation of combusion of the carbonaceous material itself to provide for recovery of the retorting by-products. Examples of such former processes are shown in U.S. Pat. No. 2,584,605, issued Feb. 5, 1952, to E. S. Merriam, et al.; U.S. Pat. No. 2,718,263, issued Sept. 20, 1955, to W. O. Heilman, et al.; U.S. Pat. No. 2,874,777, issued Feb. 24, 1959, to H. J. Tadema; U.S. Pat. No. 2,994,374, issued Aug. 1, 1961, to F. W. Crawford, et al.; U.S. Pat. No. 3,087,541, issued Apr. 30, 1963, to E. R. Elzinga; U.S. Pat. No. 3,126,954, issued Mar. 31, 1964 to F. E. Campion, and all show injection of a gas, such as air or oxygen, via a vertical shaft or drillhole as an essential factor in sustaining combustion of the carbonaceous material. Another, U.S. Pat. No. 2,801,089, issued July 30, 1957, to J. W. Scott, Jr., calls for injection of a combustible gas mixture, that includes air or oxygen, via vertical boreholes or shafts or boreholes located at the bottom of the target formation, and so is also unlike the present invention.
Other earlier art that involves tar sand heating including U.S. Pat. No. 3,048,221, issued Aug. 7, 1962, to M. R. Tek, have required generation of vertical and horizontal fractures that intersect vertical production and injection wells, with combustion of carbonaceous material supported by injection of air via injection wells. This art also teaches recovery of retorted by-products by an enhanced formation permeability as provided by the artificially generated fractures. Another, U.S. Pat. No. 3,263,750, issued Aug. 2, 1966, to W. C. Hardy, teaches that retorting efficiency of tar sand formations and subsequent recovery of by-products may be significantly enhanced by injecting, via vertical wellbores, slugs of low viscosity oil with tailored boiling points such that subsequent heating of the formation, via vertical injection wells, and maintenance of combustion by injection of air, to preclude formation of oil blocks in the tar sand formation. Also, U.S. Pat. No. 2,914,309, issued Nov. 24, 1959, to G. J. W. Salomonsson, teaches uniform heating of a tar sand formation by use of moveable heaters suspended in vertical wellbores. This patent claims that the efficiency of a retorting process is enhanced by injection of air, via vertical wellbores, to sustain combustion of a portion of the carbonaceous material contained within a target formation.
All the above-cited processes for in-situ retorting of tar sand formations require that a burn front move through the formation. Therefore, they all suffer from the common deficiency of failing to insure that air, oxygen or other gases, required to drive the process by supporting combusion of the carbonaceous material contained within the formation, are uniformly distributed within the formation and are therefore unlike the process of the present invention.
The two-stage process disclosed in the present invention represents a significant departure from prior art within our knowledge and is a significant departure from that taught in the public domain technical literature pertaining to so-called mine assisted in-situ processing (MAISP) that have as an objective to thermally mobilize bitumen using horizontal subterranean tunnels as heating conduits. Such art includes the arrangement of a U.S. Pat. No. 4,196,814, issued Aug. 15, 1978, to G. B. French, and is as detailed in a technical paper presented to a 13th Canadian Rock Mechanics Symposium in Toronto, Canada, held May 28 and 29, 1980, by D. W. Develny and J. M. Raisbeck, entitled "Rock Mechanics Considerations for In-Situ Development of Oil Sands". Rather, unlike prior processes, the present invention provides for heating via stationary line sources within the formation so as to provide a convective heat transfer system that is maintained by generation of volatiles derived from the pyrolysis of mobilized bitumen adjacent to the line source heaters. The pyrolysis zone, produced by operation of stationary line source heaters, is quasi-stationary, where as in other in-situ retorting processes, a burn front, and pyrolysis zone, are propagated through the tar sand formation. Establishment of a forced convective heat transfer system leads to further reduction of the viscosity of the hot mobilized bitumen by the solvent action of convecting thermally cracked low viscosity oils and their condensible vapors and creation and continual growth, both laterally and upward, of a high permeability zone of sand-coke. Recovery of liquid and gaseous retorting by-products so produced is preferably accomplished via an array of designated vertical boreholes.
The high permeability zone of sand-coke that builds up in the thermal cracking process is then utilized as a heat source in a second stage process of the present invention where air-supported combustion thereof is accomplished by air injection via the line source heating duct. That air injection provides a significant economic incentive with respect to diminished energy requirements for continuous operation of line source heaters and is controlled to just burn the sand-coke zone whereupon the air injection is discontinued and heating at the stationary line source is resumed.
In other in-situ processes, unlike that of the present invention, the efficiency of thermal cracking of tar is relatively low due to the inability to control the flow of air or oxygen to the combustion zone and the inability to control combustion kinetics of the virgin bitumen. With the present invention, bitumen which passes into the zone of pyrolysis, as defined by the appropriate temperature-pressure relationship and specific characteristics of the bitumen, is converted to thermally cracked by-products with an efficiency of greater than ninety percent (90%).